Download Middleware for Distributed Systems

Middleware for Distributed Systems
Old pals and the new kid(s) on the
block
Walter Kriha
What is Middleware?
software that helps two separate systems
communicate seamlessly.
(www.knownow.com/middleware/lexicon.html)
In a strict sense middleware is transport
software that is used to move information from
one program to one or more other programs,
shielding the developer from dependencies on
communication protocols, operating systems
and hardware platform (“plumbing”)
(www.talarian.com)
The Transparency Dogma
• Middleware is supposed to hide remote-ness
and concurrency by hiding distribution
behind local programming language
constructs
Critique: Jim Waldo, SUN
Full transparency is impossible and the price is too high
Where do we find Middleware?
Distributed
Cache
LDAP
or DCE
JMS
Directory
JNDI
Application
Application
Web
Server
Server
Server
WebEJB
RMI
Tier
Tier
CORBA
Part of a Portal running on a
Web Cluster.
Quotes
WebService
JDBC
XML-RPC
News
E-bank
Classification
•
•
•
•
Remote Procedure Calls (RPCs)
Object Request Brokers (ORBs)
Message Oriented Middleware (MOMs)
Others
The “ilities”
•
•
•
•
•
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Reliability
Availability
Security
Scalability
Quality
Performance
Maintainability
Before using a specific
middleware, always make sure
that the “ilities” aka nonfunctional requirements are
met. Middleware almost
always differs implementation
quality between vendors.
Real-World Problems
• Skills/Understanding: Best practice
patterns?
• Single-Point-Of-Failures: replication, loadbalancing etc.
• Tooling: generators, deployment tools
• “Brittle-ness” if interfaces change
(Compiler “illusion”)
RPC type Middleware
• E.g. Sun-RPC, OSF DCE
• Main idea: distribute functions, use
concurrent processing
• On top of it: Distributed Directory, File
system, Security (cells, principals)
• XML-RPC over http (www.userland.com)
Layer foundations: UUIDs, value vs. reference, marshaling,
versioning etc.
Distributed Objects
CORBA
RMI
• Object Request Broker
• Multi-language support
(platform independence)
• Interface Definition
language
• Wire Protocoll: IIOP,
GIOP
• Java only (e.g.Introspection
used)
• Lightweight method call
semantics
• Java Implementations
• Wire Protocoll now mostly:
RMI over IIOP
Both try to preserve object semantics. Interface/Implementation
separation
Distributed Components
• Objects are too granular: performance and
maintenance problems
• Programmers need more help: separation of
concerns and context
Solutions:
• Enterprise Java Beans
• CORBA Components
• COM+
Example: Enterprise Java Beans
EJB Framework (Separation
of concerns):
Deployment (Separation of
context):
Automatic Transaction
Management
System Management defines Data
Sources and Containers
Concurrency Control
System Management defines Pool sizes
Automatic, method level Security
System Management defines
Role/User Binding
EJB Container
invoke
Client
Entity Bean
Load/
persist
delegate
At the point of
interception the
container provides
the following
services to the bean:
Resource management, life-cycle, statemanagement, transactions, security, persistence
Distributed Messages (MOM)
Asynchronous, loosely-coupled (fault tolerant), persistent
messages with either publish/subscribe (topics) or queuing
semantics. Scales well. Delivery guarantees differ.
Pub
Topic
publish
send
Sub
Pub
Sub
Put
(M1,M2)
Get
M2 M1
queue
Sub
Get
MOM
Sub
MOM
Sub
Distributed Code I (Agents, Aglets)
The Problem: who wants a new runtime system?
Agent
pack
Agent
Runtime
OS
Agent
Serialized Agent unpack
Channel
Agent
Runtime
OS
Perform
work,
come
back
with
results
Distributed Code II (Jini) – The
End of Protocols?
Proxy moves to
client during
service lookup
Jini Client
Jini Lookup
Service
Service private
protocol
Service Proxy Code
Proxy moves to
lookup service
during
registration
Jini Service
Peer 2 Peer
Seti@home, freenet
JXTA etc.
Nodes have no
fixed IP address and
frequent downtimes
INTERNET
ISP DNS
ISP
ISP
Problems: How do you version
files? Overhead?
P2P uses cycles,
provides file sharing
and anonymity because
no central servers are
used
WebServices
Promises de-coupling of service provider and requester,
document interfaces, machine-to-machine communication
and ease of use compared to distributed objects. No way to
define QOS or conversation yet.
Security, Transactions etc.
Core services
Universal Description, Discovery and Integration Registry (advertise)
Web Services Description Language
SOAP
XML Syntax/HTTP
Web Server
Service features
exchange messages
Wire Format/
Transport
Broker
Service Granularity? Application, Component, Object or Request?
Use your “de-hyper” generously!
Others
• Parallel Processing:
PVM, MPI (e.g. for
Linux Beowulf
cluster)
• Wireless:
Bluetooth
• System Management:
Jiro/FMA
• Linda Tuple Spaces
(Javaspaces, Tspaces)
• Group Computing
(virtual synchrony):
Horus, iBus
Resources
• Jim Waldo, End of
Protocols
• Marco Boger,Java in
verteilten Systemen
• Ken Birman, Building
secure and reliable
Network Application
• ObjectSpectrum 7/2001,
WebServices
• Java Magazine 7/2001,
Java Message Service
• www.theserverside.com
on EJBs
• Clay Shirky, What is P2P
and what Isn’t
(www.openp2p.com)
• S.Tai, I.Rouvellou,
Strategies for Integrating
Messaging and Distributed
Object Transactions